Spark plug with improved auxiliary spark gap



R. w. SMITH 2,988,662

SPARK PLUG WITH IMPROVED AUXILIARY SPARK GAP June 13, 1961 Filed Dec 18, 195? United States Patent M. SPARK PLUG WITH IMPROVED AUXILIARY SPARK GAP Robert W. Smith, Flint, Mich, assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware Filed Dec. 18, 1957, Ser. No. 703,559 9 Claims. (Cl. 313-424) This invention relates to an improved spark plug of the type having an auxiliary spark gap incorporated in the center electrode.

It has long been known that an auxiliary spark gap in series with the firing gap of a spark plug can function to control the breakdown voltage at the firing gap and enable the plug to function efficiently despite accumula tions of carbon or the like in the firing gap which would otherwise cause fouling. However, this advantage has not been attained to its fullest extent with prior auxiliary gap constructions principally because they have not been capable of providing the desired constancy of breakdown voltage over a long period of operation. That is, in prior auxiliary gap constructions, the breakdown voltage of the auxiliary gap itself has varied as operation continued. Another serious disadvantage of prior auxiliary gap-type spark plugs has been the expense involved in their manufacture; accurate spacing of the auxiliary gap electrodes has been a relatively costly operation.

It is one of the objects of the present invention to provide a spark plug with an improved auxiliary gap construction which retains its desired electrical characteristics even after prolonged periods of operation. Another object of the invention is the provision of an improved auxiliary gap type spark plug which may be manufactured at low cost. These and other objects are accomplished, in accordance with the invention, by an auxiliary spark gap constructed of two electrodes having interposed therebetween a compacted mass of alumina grain. The electrodes of the auxiliary gap preferably consist of a layer of compacted high heat and erosion resistant metal powder, all as will now be more fully described with reference to the appended drawing which shows a side view in partial section of a preferred embodiment of the invention.

Referring now to the embodiment shown in the drawing, 2 is a metal shell and 4 an insulator secured within the shell in the conventional manner. The insulator 4 is formed with a centerbore having a lower portion 6 of relatively small diameter, a center portion 8 of larger diameter and a top portion 10 of still larger diameter which serves as a well for the reception of an electrical cable (not shown) from the ignition circuit. In the lower portion 6 of the centerbore there is secured a center electrode 12, the lower end of which extends beyond insulator 4 into spaced relationship with ground electrodes 14 to form the firing gap of the spark plug. The upper end of electrode 12 is formed with a head portion 16 which cooperates with internal annular shoulder 17 of the insulator to retain the electrode in its fixed position. In the particular embodiment shown, theelectrode member 12 is the type covered by United States Patent 2,615,441, issued October 28, 1952, to W. A. Bychinsky. However, other electrode structures may be used if desired.

In the upper end of portion 8 of the centerbore is a metal pin or terminal 18 having a head portion 20 which abuts the insulator shoulder 22 and serves as a contact to form the electrical connection for the spark plug. The centerbore is hermetically sealed by electrically conductive glass seals 24 and 26 which are bonded tothe insulator and to metal members 12 and 18, respectively.

The lower end of pin member 18 which is embedded Patented June 13, 196- 1:

in the conductive glass seal 26 is formed with threads in order to provide additional surface area for a good bond. The conductive glass seals consist of a mixture of glass and powdered metal such as nickel or copper, and may be advantageously formulated and formed in accordance with United States Patent 2,248,415, granted July 8, 1941, to Schwartzwalder and Rulka.

Between glass seals 24 and 26 is the auxiliary spark gap formed, in accordance with the invention, by layers 28 and 30 of compacted nickel powder, having interposed therebetween layer 32 of compacted alumina grain. The nickel powder layers 28 and 30 are in intimate contact with conductive glass layers 24 and 26, respectively, and serve as the electrodes of the auxiliary gap. Compacted alumina grain 32 which fills the gap between the nickel powder electrodes serves to provide constancy of break-1 down voltage throughout the life of the plug, an advantage not attainable with an air gap or with other dielectric materials such as glass or mica. The alumina grain preferably should have a grain size of from about 48 to mesh. For the nickel powder a grain size of 100 mesh or finer is very satisfactory. A mixture of 100 mesh and fines has the advantage of packing well,'the smaller particles fitting between the large.

An important advantage of the invention is low cost, this because of the ease and simplicity with which the auxiliary gap may be formed in accordance, with the following method.

After the ceramic insulator is formed and fired by conventional methods, the electrode 12 is positioned in the centerbore and then successive, separate measured charges of conductive glass seal mixture, nickel powder, alumina grain, nickel powder and conductive glass seal mixture are poured into the bore in the order indicated; the addition of each separate charge being followed by a tamping or similar operation to provide a level layer. The leveling operation is not essential for all layers, but it is for the lower nickel layer and the aluminagrain. layer, this in order to obtain an auxiliary gap of uniform thickness. After the measured charges have been added, element 18 is positioned in the centerbore and the entire assembly fired at about from 1600 to 1700" F. for approximately 20 minutes in order to soften the conductive glass seal mixture. Then, while the assemblyis heated, a downward force of about from 100 to 200 pounds is applied to element 18 in order to compress the layers, the lower end of element 18 thereby becoming embedded in seal 26. During the firing operation the nickel powder layers 28 and 30 will sinter to some extent. The remaining operations to make the finished spark plug may be by conventional method-s.

Because the auxiliary gap can be formed by hopperfed powder charges, it can be manufactured at low cost, precise control of gap size being accomplished relatively inexpensively by merely controlling the amount of alumina grain added. The powdered metal electrodes are particularly advantageous not only because they can be hopper-fed and therefore lend to the simplicity of marine facture, but also because intimate and continuous cone tact between the electrodes and the alumina grain is thereby assured. v I

It is to be understood that, although the inventionhas been described with specific reference to a particular. embodiment thereof, it is not to be so limited since changes and alterations therein may be made which are within the full and intended scope of the claims which follow.

1. A spark plug comprising a metal shell having a ground electrodesecured to one end thereof, an insulator having a centerbore ther'ethro'ugh secured within said shell, a center electrode in one end of said centerbore having a head portion formed at the upper end thereof 3 and a lower portion extending to the exterior of said insulator into spaced relationship with said ground electrode to form a spark gap therewith, a conductive sealing layer in said centerbore above and bonded to the head portion of said center electrode, a compacted layer of high heat and erosion resistant metal powder in said centerbore above and in contact with said conductive sealing layer, a compacted layer of alumina grain in said centerbore above and in contact with said metal powder layer, a second compacted layer of high heat and erosion resistant metal powder in said centerbore above and in contact with said alumina grain, and a second conductivelsealing layer in said centerbore above and in contact with said second metal powder layer. 2. A spark plug comprising a metal shell having a ground electrode secured to one end thereof, an insulator having a centerbore therethrough secured within said shell, a center electrode in one end of said centerbore having a head portion formed at the upper end thereof and a lower portion extending to the exterior of said insulator into spaced relationship with said ground electrode to form a spark gap therewith, a conductive sealing layer of a mixture of glass and metal powder in said centerbore above and bonded to the head portion of said center electrode, a compacted layer of nickel powder in said centerbore above and in contact with said conductive sealing layer, a compacted layer of alumina grain in said centerbore above and in contact with said nickel powder laye'r,'a second compacted layer of nickel powder in said centerbore above and in contact with said alumina grain, a second conductive sealing layer of a mixture of glass and metal powder in said centerbore above and in contact with said second nickel powder layer, and a metal contact element in said centerbore having a lower portion embedded in said second conductive sealing layer.

3. A spark plug comprising a metal shell having a ground electrode secured to one end thereof, an insulator having a centerbore therethrough secured within said shell, a center electrode in one end of said centerbore having a head portion formed at the upper end thereof and a lower portion extending to the exterior of said insulator into spaced relationship with said ground electrode to form a spark gap therewith, a conductive sealing layer of a mixture of glass and metal powder in said centerbore above and bonded to the head portion of said center electrode, a compacted layer of nickel powder in said centerbore above and in contact with said conductive sealing layer, a compacted layer of alumina grain in said centerbore above and in contact with said nickel powder layer, a compacted second layer of nickel power in said centerbore above and in contact with said alumina grain, a second conductive sealing layer of a mixture of glass and metal powder in said centerbore above and in contact with said second nickel powder layer, and a metal contact element in said centerbore having a lower portion embedded in said second sealing layer, said alumina grain having a grain size of from about 48 to 100 mesh and said nickel powder having a grain size of 100 mesh and finer.

4. A spark plug comprising a metal shell having a ground electrode at one end thereof, an insulator hav- .ing a centerbore therethrough secured within said shell,

an electrical terminal in one end of said centerbore, a center electrode in the other end of said centerbore extending into spaced relationship to said ground electrode to form a spark gap therewith, and an auxiliary spark gap in said centerbore formed by a pair of metal members spaced fromeach other by a compacted mass of grain consisting of alumina, one of said metal members being electrically connected to said terminal and the other of said metal members being electrically connected to said center electrode.

5. A spark plug comprising a metal shell having a ground electrode'at one end thereof, an insulator having a centerbore therethrough secured within said shell, an electrical terminal in one end of said centerbore, a center electrode in the other end of said centerbore extending into spaced relationship to said ground electrode to form a spark gap therewith, and an auxiliary spark gap in said centerbore formed by two compacted layers of heat and erosion resistant metal powder spaced from each other by a compacted mass of grain consisting of alumina, one of said layers being electrically connected tosaid terminal and the other of said layers being electrically connected to said center electrode.

6. A spark plug comprising a metal shell having a ground electrode at one end thereof, an insulator having a centerbore therethrough secured within said shell, an electrical terminal in one end of said centerbore, a center electrode in the other end of said centerbore extending into spaced relationship to said ground electrode to form a spark gap therewith, a conductive layer of a mixture of glass and metal powder in said centerbore bonded to the walls thereof to form a seal and bonded into electrical connection with said center electrode, and an auxiliary spark gap in said centerbore formed by a pair of metal members spaced from each other by a compacted mass of grain consisting of alumina, one of said metal members being electrically connected to said terminal and the other of said metal members being electrically connected to said conductive layer.

7. An electrical discharge device comprising an insulator having a centerbore therethrough, an electrical terminal in one end of said centerbore, an electrode in the other end of said centerbore and an auxiliary spark gap in said centerbore formed by a pair of metal members spaced from each other by a compacted mass of grain consisting of alumina, one of said metal members being electrically connected to said terminal and the other of said metal members being electrically connected to said electrode.

8. An electrical discharge device comprising an insulator having a centerbore therethrough, an electrode in one end of said centerbore having a head portion formed at the upper end thereof and a lower portion extending to the exterior of said insulator, a conductive sealing layer in said centerbore above and bonded to the head portion of said center electrode, a layer of high heat and erosion resistant metal in said centerbore above and in contact with said conductive sealing layer, a compacted layer of alumina grain in said centerbore above and in contact with said metal layer, a second layer of high heat and erosion resistant metal in said centerbore above and in contact with said alumina grain, and a second conductive sealing layer in said centerbore above and in contact with said second metal layer.

9. An electrical discharge device comprising an insulator having a centerbore therethrough, a center electrode in one end of said centerbore having a head portion formed at the upper end thereof and a lower portion extending to the exterior of said insulator, a conductive sealing layer of a mixture of glass and metal powder in said centerbore above and'bonded to the head portion of said center electrode, a nickel layer in said centerbore above and in contact with said conductive sealing layer, a compacted layer of alumina grain in said centerbore above andin contact with said nickel layer, a second nickel layer in said centerbore above and in contact with said alumina grain, a second conductive sealing layer of a mixture of glass and metal powder in said centerbore above and in contact with said second nickel layer, and a metal contact element in said centerbore having a lower portion embedded in said second conductive sealing layer.

References. Cited in the file of this patent UNITED STATES PATENTS 2,029,570 Kasarjian Feb. 4, 1936 2,356,104 'Tognola Aug. 15, 1944 

